A semiconductor photosensing array is often used in instruments such as spectrophotometers and gas chromatographs to measure the spectrum of ultraviolet, visible and near infrared light. A large number of closely spaced photosensing elements are positioned to form an array and a beam of incident light is dispersed with respect to wavelength by a diffraction grating or similar dispersing device. As a result of such dispersement by wavelength, light having different wavelength ranges falls on different photosensing elements in the array.
For example, light may be passed through a sample to be analyzed, whereafter the light is directed to a diffraction grating, prism or other wavelength dispersal device so that different wavelengths of light are dispersed in different spatial directions. The desired wavelength range is intercepted by the photosensing array. The accuracy of analysis is primarily dependent upon the wavelength dispersal device and upon the spatial resolution of the photosensing array. Ideally, incident light strikes the photosensing array at normal incidence and any light passing through a particular sensing element is detected only by that sensing element. However, in practice photons may reach the photosensing array at an angle other than 90.degree.. Moreover, the photosensing elements are typically arranged in a uniformly doped substrate. When light enters the uniformly doped collecting region associated with a particular photosensing element, photogenerated carriers are formed. While some of the photogenerated carriers arrive at the photosensing element through which the light passed, a photogenerated carrier is subject to random motion and can diffuse a significant distance to the collecting region of a distant photosensing element. This random diffusion causes a spurious signal to degrade the spatial resolving power of the array and has been termed "optical crosstalk".
Regarding the problem of light entering one collecting region and passing into an adjacent collecting region wherein carriers are photogenerated, the arrays are manufactured to reduce the likelihood of such occurrence. For example, the photosensing elements are spaced apart to provide a control region and the elements are covered by materials which act as a waveguide within which an optical ray arriving at an undesired angle will be reflected and eventually absorbed.
Regarding the problem of optical crosstalk, the problem is greater for penetrating long-wavelength light since photogenerated carriers created deep within the semiconductor substrate have a greater probability of diffusing to distant photosensing elements than do carriers generated close to the surface. Because of the low absorption coefficient of materials typically used in manufacture of the semiconductor substrate, e.g., Si, GaAs and Ge, a photon can penetrate and form a carrier at distances of 100 microns or more beneath the light-receiving surface of the semiconductor substrate.
Attempts have been made to reduce optical crosstalk so as to improve the spatial resolution of a photosensing array. U.S. Pat. No. 4,160,985 to Kamins et al., assigned to the assignee of the present application, teaches formation of a buried layer to produce electronic fields in the semiconductor substrate which accelerate photogenerated carriers either toward or away from the surface of the array depending upon the depth of the carriers.
It is an object of the present invention to provide a photosensing array and method of making of same which provide isolation of collecting regions both with respect to an optical ray and with respect to migration of photogenerated carriers.